Modern magnetically based electronic article surveillance systems generally derive their parentage from 1934 French Pat. No. 763,681. That patent depicts the use of markers formed of a piece of low coercive force, high permeability alloy, such as permalloy, and teaches that when the magnetization of such a piece is reversed by a magnetic field alternating at a fundamental frequency, detectable harmonics of that frequency will be produced.
More recently, various investigators have developed magnetic markers which have dual-status capabilities. Typically, as first disclosed in U.S. Pat. Nos. 3,665,449 (Elder et al.) and 3,747,086 (Peterson), such dual-status markers include at least one piece of low coercive force, high permeability material together with at least one piece of remanently magnetizable material. When the latter piece is magnetized it has associated therewith a magnetic field which biases the low coercive force, high permeability material so as to alter the signal produced when the biased material is in the interrogation field. It is also disclosed in the '449 patent that such dual-status markers may comprise coextensive strips of magnetizable material and high permeability, low coercive force material, and while not preferred, that the magnetizable material could be uniformly magnetized. That patent fails to suggest how signals from such markers in which the magnetizable strip is alternatively not magnetized or is uniformly magnetized could be reliably distinguished Similarly, one marker embodiment depicted in the '086 patent comprises two coextensive strips. While that patent indicates that magnetization of one strip alters the harmonic content of the signal produced by the other, the exact nature of the magnetization is not specified. The disclosure pertaining to FIG. 6D of the '086 patent suggests only that magnetization be such as to leave the responder strip in a fully magnetized condition, thereby causing the marker to be completely silent
While the '449 and '086 patents thus suggest that single directionally responsive markers may be deactivated by a magnetic bias field extending the full length of the responder strip, it has now become well recognized that reliable deactivation is obtained by providing discontinuous fields so that the responder strip essentially responds as a number of strips of shorter length. This is effected in typical, commercially viable systems by providing a number of magnetizable pieces spaced along the responder strip or by providing a continuous strip of magnetizable material which is magnetized in bands of alternating polarity.
More recently, multi-directionally responsive magnetic markers have also been developed. Thus, for example, as set forth in a recent patent of the present inventor, i.e., U.S. Pat. No. 4,710,754 such markers may comprise a square piece of low coercive force, high permeability material fabricated to have regions with narrow widths centered along each edge of the squares, thereby providing switching sections and extensive regions in each corner which collect and channel flux into the switching sections. The markers of the '754 patent are made dual-status by adding discrete pieces of magnetizable material adjacent each switching section.
A further embodiment of a dual-status, multi-dimensionally responsive marker is disclosed in U.S. Pat. No. 4,825,194 (Church et al.) in which discrete magnetizable pieces are positioned adjacent flux collector sections of a sheet of responder material Optionally, that patent also suggests that additional pieces of magnetizable material may be positioned adjacent the switching sections, but that the separation between the respective magnetizable pieces be sufficient to prevent appreciable magnetic coupling therebetween.
Multi-dimensionally responsive markers in which a coextensive sheet of magnetizable material is provided together with a sheet of low coercive force, high permeability responder material are disclosed in a second, recent patent of the present inventor, i.e., U.S. Pat. No. 4,746,908. However, the markers of the '908 patent function in a significantly different manner and utilize a piece of responder material configured so as not to create a desired response The coextensive sheet of magnetizable material is magnetized with a predetermined pattern which biases only adjacent portions of the responder material, thereby inhibiting response from those portions. The magnetized pattern is such that the dimensions of the unbiased, remaining portion can then produce the desired response. Such markers thus function oppositely to those in typical use, i.e., that the marker is magnetized when in its sensitive state.